Add `Time::Span` for `interval` column

in postgres, you can do this

```
# select interval '1 hour';
interval
----------
01:00:00
(1 row)
```

and in Crystal `1.hour` gives you `01:00:00`.

No files or tests are named. Start by locating Avram's PostgreSQL column-type handling and the existing time-related mappings; then determine how an interval column should map to Crystal's Time::Span. Done means interval columns are supported consistently and covered by tests for reading and writing values.

Written by the indexing model from the issue text.